A/d converter circuit and solid-state imaging device

ABSTRACT

Provided are a solid-state imaging device and A/D converter circuit comprising: series-connected capacitative elements; a voltage comparator circuit comparing the output of the capacitative element C 1  with a threshold voltage; a first input circuit inputting an analog voltage signal to the node between the capacitative elements C 1  and C 2 ; a second input circuit inputting a first reference voltage, monotonously changing in a first conversion process for finding the upper-order bit value, to the node between the capacitative elements C 2  and C 3 ; a third input circuit inputting a second reference voltage, monotonously changing in a second conversion process for finding an unconverted bit value after the first conversion process, to the input terminal of the capacitative element C 3 ; and a control circuit generating a control signal to hold the first reference voltage in the capacitative element C 3  when the output of the voltage comparator circuit changes in the first conversion process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase filing under 35 U.S.C. §371 ofInternational Application No. PCT/JP2009/051534 filed on Jan. 30, 2009,and which claims priority to Japanese Patent Application No. 2008-027689filed on Feb. 7, 2008.

TECHNICAL FIELD

The present invention relates to an A/D converter circuit to convert ananalog signal to a digital signal and more particularly, to acolumn-parallel A/D converter circuit provided in a solid-state imagingdevice in which a plurality of solid-state imaging elements each toconvert an optical signal to an electric signal are arranged in the formof a matrix, and mounted on the same chip as the solid-state imagingelements.

BACKGROUND ART

On some solid-state imaging devices each provided with a solid-stateimaging element group in which a plurality of solid-state imagingelements (unit pixel) to convert an optical signal to an electric signalby photoelectron conversion are arranged in the form of a matrix, suchas a CMOS imaging sensor, CCD sensor, near-infrared imaging sensor, orfar-infrared imaging sensor, an analog circuit and a digital circuit aremounted on the same chip.

The circuit mounted on the same chip as the solid-state imaging elementgroup includes, for example, a column-parallel A/D converter circuit inwhich an A/D converter is provided with respect to each column of thesolid-state imaging element group, and pixels in one row are read at onetime. Since the column-parallel A/D converter circuit can read the datain one row at one time, its reading speed is high as compared with aconventional A/D converter which is not the column-parallel A/Dconverter, or its reading speed can be at the same level as theconventional A/D converter which is not the column-parallel A/Dconverter, at a lower operation frequency, so that power consumption canbe lowered.

The general column-parallel A/D converter circuit includes, for example,a column-parallel A/D converter circuit (refer to patent document 1, forexample) which is composed of a ramp voltage generator circuit togenerate a reference voltage (ramp voltage) whose voltage valuemonotonously increases in a conversion process to convert an analogvoltage signal outputted from the solid-state imaging element to digitaldata, and a counter circuit to output a counted digital value inresponse to a voltage change of the ramp voltage, and compares theanalog voltage signal with a reference voltage signal in synchronizationwith an counting operation of the counter circuit, and stores a countervalue as pixel data when a comparison result is inverted.

A brief description will be made of a configuration of a solid-stateimaging device provided with the column-parallel A/D converter disclosedin the patent document 1 with reference to FIG. 10. Here, FIG. 10partially shows a schematic configuration example of the solid-stateimaging device provided with the column-parallel A/D converter circuitdisclosed in the patent document 1. In addition, it is assumed that aresolution (defined by the bit number) is 10 bits in the column-parallelA/D converter circuit shown in FIG. 10.

More specifically, as shown in FIG. 10, a solid-state imaging device1000 disclosed in the patent document 1 is composed of a solid-stateimaging element group IPD in which a plurality of solid-state imagingelements PIX_(ij) (i=1 to m, j=1 to n) to convert an optical signal toan analog voltage signal Vpix are arranged in the form of a matrix, avertical decoder VD to select a reading target row in a reading process,a ramp voltage generator circuit 1020 to generate a ramp voltage Vrwhose voltage value increases step-by-step according to a counter valueof a counter circuit 1040 to be described below in a conversion processto convert the analog voltage signal Vpix to digital data, the countercircuit 1040 to start counting when the voltage value of the rampvoltage Vr starts increasing, a horizontal decoder HD to select areading target column in the reading process, a converter circuit group1100 in which converter circuits 1101 to compare the analog voltagesignal Vpix with the ramp voltage Vr, and output a signal Vcp′ showing acomparison result are each provided with respect to each column of thesolid-state imaging element group IPD, and a digital memory 1050 inwhich memory circuits 1051 are each provided with respect to each columnof the solid-state imaging element group IPD. In addition, thecolumn-parallel A/D converter circuit ADC is composed of the rampvoltage generator circuit 1020, the counter circuit 1040, the convertercircuit group 1100, and the digital memory 1050.

The converter circuit 1101 is composed of a capacitative element 1105having an input end to which the analog electric signal Vpix is inputtedthrough a switch circuit 1102, a capacitative element 1104 having anoutput end connected to an intermediate node connecting the switchcircuit 1102 to the capacitative element 1105 and an input end to whichthe ramp voltage Vr is inputted through a switch circuit 1103, a voltagecomparator circuit 1106 composed of an inverter circuit having an inputterminal connected to an output end of the capacitative element 1105 tocompare a voltage value of the input terminal with a predeterminedthreshold voltage value, and a switch circuit 1107 to cause shortcircuit between an input terminal and an output terminal of the voltagecomparator circuit 1106.

Hereinafter, a description will be made of an operation of thecolumn-parallel A/D converter circuit ADC in the solid-state imagingdevice 1000 with reference to FIGS. 11 and 12.

Here, FIG. 11 shows waveforms of input and output voltage signals andoperation states of switch circuits SW1′ to SW3′ in the convertercircuit 1101 of the column-parallel A/D converter circuit ADC of thesolid-state imaging device 1000 shown in FIG. 10. FIG. 12 shows arelationship among values of the ramp voltage Vr, and analog signalVpix, and the counter circuit 1040 while the conversion process isexecuted. In addition, FIG. 12 shows a case where a resolution of thecolumn-parallel A/D converter circuit ADC is 4 bits, the counter circuit1040 counts 2⁴=16 times in a period Trc, and the voltage value of theramp voltage Vr increases step-by-step by a unit increase amount inresponse to the counting operation. In addition, in FIG. 12, SW1′ showsthe state of the switch circuit 1102, SW2′ shows the state of the switchcircuit 1103, and SW3′ shows the state of the switch circuit 1107.

When an imaging process is started at a time t0, an initializationprocess is started at a time t1. In the initialization process, avoltage Vrst of the solid-state imaging element PIX at a reset level issampled.

More specifically, as shown in FIG. 11, a voltage value of theconversion object analog voltage signal Vpix outputted from thesolid-state imaging element PIX is equal to the voltage Vrst at thereset level at the time t1, and a voltage value of the ramp voltage Vris equal to a voltage Vr0 at an initial level. When the initializationprocess is started at the time t1, the converter circuit 1101 of thecolumn-parallel AJD converter circuit ADC turns on the switch circuit1102 and the switch circuit 1107, and turns off the switch circuit 1103.Thus, the voltage Vrst of the solid-state imaging element PIX_(ij) atthe reset level is inputted to the input end of the capacitative element1105 (C1′), and short circuit is caused between the input terminal andthe output terminal of the voltage comparator circuit 1106, and aninversion level Vth of the voltage comparator circuit 1106 is inputtedto the output end of the capacitative element 1105 (C1′). Thus,characteristic variation in inversion level of the voltage comparatorcircuit 1106 is cancelled (auto-zero technique). Then, when the switchcircuit 1107 is turned off at a time t2, a difference voltage betweenthe voltage Vrst of the solid-state imaging element PIX_(ij) at thereset level and the inversion level Vth of the voltage comparatorcircuit 1106 is held in the capacitative element 1105 (C1′), and theinitialization process is completed.

At a time t3, a sampling process is started for a signal level Vsig ofthe solid-state imaging element PIX_(ij) in an image loading process.

More specifically, at the time t3, the conversion object analog voltagesignal Vpix whose voltage value is the voltage Vsig is outputted fromthe solid-state imaging element PIX_(ij), and inputted to the input endof the capacitative element 1105 (C1′). In addition, at the time t3, theswitch circuit 1103 (SW2′) is turned on, and the ramp voltage Vr at theinitial level Vr0 is inputted to the input end of the capacitativeelement 1104 (C2′). At a time t4, the sampling process for the signallevel Vsig of the solid-state imaging element PIX_(ij) is completed.When the switch circuit 1102 (SW1′) is turned off at the time t4, adifference voltage between the signal level Vsig of the conversionobject analog voltage signal Vpix and the initial level Vr0 of the rampvoltage Vr is held in the capacitative element 1104 (C2′).

At a time t5, the conversion process to convert a difference voltage Vato digital data is started, and the ramp voltage Vr increasesstep-by-step in synchronization with a clock signal CLK, and the countervalue of the counter circuit 1040 increases by one in response to theincrease of the ramp voltage Vr.

More specifically, as shown in FIG. 12, the voltage value of thedifference voltage Va is a value between the voltage value of the rampvoltage Vr corresponding to the counter value “1000” and the voltagevalue of the ramp voltage Vr corresponding to the counter value “1001”.The counter circuit 1040 sequentially counts up from “0000” by one, andwhen the counter value changes from “1000” to “1001” at a time t6, avalue of the signal Vcp′ outputted from the voltage comparator circuit1106 is changed (inverted) from H level to L level. When the value ofthe signal Vcp′ is inverted, the memory circuit 1051 stores the countervalue of the counter circuit 1040, that is, “1001” in FIG. 12. Then, theconversion process is completed at a time t7.

The following formula 1 and formula 2 express an input voltage Vin ofthe voltage comparator circuit 1106 in the column-parallel A/D convertercircuit ADC disclosed in the patent document 1 with the inversion levelVth of the comparator circuit 1106, the initial level Vrst of thesolid-state imaging element PIX_(ij), the signal level Vsig of thesolid-state imaging element PIX_(ij) at the time of imaging operation,and the initial level Vr0 of the ramp voltage Yr.

Vin=Vr+(Vsig−Vr0)+(Vth−Vrst)  (1)

Vin−Vth=Vr−{Vr0+(Vrst−Vsig)}  (2)

Therefore, based on the formula 1, the formula 2, and FIG. 11, the rampvoltage Vr at the time of the output change of the voltage comparatorcircuit 1106, that is, when Vin-Vth=0 is expressed by the followingformula 3.

Vr=Vr0+(Vrst−Vsig)=Vr0+Va

(provided that Vin−Vth=0)  (3)

In addition, in the case of the column-parallel A/D converter circuitADC disclosed in the patent document 1, the conversion process toconvert the conversion object analog voltage signal Vpix to digital dataD [(z−1):0] (z is the bit number of the digital data) requires a timefor 2^(z) steps, that is, for the 2^(z) clocks because one stepcorresponds to one clock in general. More specifically, when the bitnumber z=10 in the digital data, a time for 2¹⁰=1024 clocks is required.

By the way, recently, the solid-state imaging device provided with thecolumn-parallel A/D converter circuit is required to be higher inresolution and higher in resolution accuracy, so that various techniquesare proposed to shorten a process time of the column-parallel A/Dconverter circuit.

The technique to shorten the process time of the column-parallel A/Dconverter circuit includes a sub-ranging method in which the digitaldata is divided to upper-order bits and lower-order bits, and thedigital data is provided by a two-step conversion process composed of afirst conversion process to convert the conversion object analog voltagesignal to the digital data with a upper-order bit converting resolutionbased on the bit number of the upper-order bits, and a second conversionprocess to convert a difference voltage value between a voltage valuecorresponding to the conversion result of the first conversion processand a voltage value of the conversion object analog voltage signal, tothe digital data with a lower-order bit converting resolution based onthe bit number of the lower-order bits.

Hereinafter, a description will be made of a concept of a generaloperation of a conventional sub-ranging A/D converter circuit withreference to FIGS. 8 and 9. Here, FIG. 8 shows a relationship among aconversion object analog voltage signal Vpix, a voltage valuecorresponding to the upper-order bits found in the first conversionprocess, a difference voltage Vdif, and a voltage value corresponding tothe digital data, in the general sub-ranging A/D converter circuit. FIG.9 shows configurations of a first ramp voltage Vrc used in the firstconversion process and a second ramp voltage Vrf used in the secondconversion process. In addition, in FIGS. 8 and 9, it is assumed thatthe digital data is composed of 4 bits, and the first conversion processto find the upper-order 2 bits and the second conversion process to findthe lower-order 2 bits are executed to simplify the description. Inaddition, a voltage Ver shown in FIG. 8 represents a quantizing error atthe time of the A/D conversion process.

As shown in FIGS. 8 and 9, in order to find the upper-order 2 bits, thefirst conversion process uses the ramp voltage Vrc which monotonouslyincreases by a first change amount Δ1 provided by dividing a voltagewidth Vfs of the conversion object analog voltage signal Vpix by splitnumber 4 based on the resolution of 2 bits. In FIGS. 8 and 9, theconversion object analog voltage signal Vpix is provided between digitalvalues “01” and “10” of the counter circuit, so that when the digitalvalue of the counter circuit (corresponding to the counter circuit 1040in FIG. 10) shifts to “10”, an output value of the voltage comparatorcircuit (corresponding to the voltage comparator circuit 1106 in FIG.10) is inverted. The counter value “10” at this time is the value of theupper-order bits, and the difference voltage value Vdif at this time isheld as a process target of the second conversion process.

As shown in FIGS. 8 and 9, in order to find the lower-order 2 bits, thesecond conversion process uses the ramp voltage Vrf which monotonouslydecreases by a second change amount ΔV2 provided by dividing the firstchange amount ΔV1 by split number 4 based on the resolution of 2 bits.In FIGS. 8 and 9, the difference voltage value Vdif is provided betweenthe digital values “10” and “01”, and when the counter value shifts to“01” in the counter circuit, the output value of the voltage comparatorcircuit is inverted. The counter value “01” at this time is the value ofthe lower-order bits, whereby it is found that digital data D[(z−1):0]=“1001”.

The sub-ranging column-parallel A/D converter circuit includes, for anexample, a column-parallel A/D converter circuit internally containing adifference detection circuit (refer to a patent document 2, forexample), as shown in FIG. 13, which is provided with a conversioncircuit 2000 having a sampling and holding circuit (hereinafter, simplyreferred to as the “S/H circuit” occasionally) 2001 to hold theconversion object analog voltage signal Vpix, a comparator 2002 tocompare the conversion object analog voltage signal Vpix with the rampvoltage Yr (=upper-order bit converting ramp voltage Vrc), a differencedetection circuit 2004 to output difference voltage value between theconversion object analog voltage signal Vpix and the ramp voltage Vrc, aS/H circuit 2005 to hold the voltage value outputted from the differencedetection circuit 2004, a logic circuit 2003 having a function tocontrol and make the S/H circuit 2005 hold the difference voltage valueat the time of the output change of the comparator 2002, an attenuationcircuit 2006 to generate the ramp voltage Vrf in which a voltage widthof the ramp voltage Vrc is adjusted to 1/K, a comparator 2007 to comparethe ramp voltage Vrf with the voltage held in the S/H circuit 2005, anda logic circuit 2008 to generate a signal Vcp_1′ to define a timing forholding the value of the counter circuit corresponding to the value ofthe upper-order bits, based on the output signal of the comparator 2007.

In addition, in the case of the column-parallel A/D converter circuitdisclosed in the patent document 2, when it is assumed that the bitnumber of the digital data is 10, the bit number of the upper-order bitsis 5, and the bit number of the lower-order bits is 5, a time for 2⁵=32clocks is required for the first conversion process, and a time for2⁵=32 clocks is required for the second conversion process, so that atime for 64 (=32+32) clocks is required. Therefore, as described above,while column-parallel A/D converter circuit disclosed in the patentdocument 1 to convert the conversion object analog voltage signal to thedigital data by the one-step conversion process needs the time for 1024clocks, the sub-ranging column-parallel A/D converter circuit disclosedin the patent document 2 only needs the time for 64 clocks in theconversion process, so that the process time is considerably shortened.

As another sub-ranging column-parallel A/D converter circuit, there is acolumn-parallel A/D converter circuit using a capacity ratio (refer to apatent document 3, for example), as shown in FIG. 14, composed of a S/Hcircuit 3001 to hold the conversion object analog voltage signal Vpix, acomparator circuit 3005 to compare the conversion object analog voltagesignal Vpix with a reference voltage, a switch circuit 3002 to input theramp voltage Vrc to the comparator circuit 3005 at the time of theexecution of the first conversion process to find the upper-order bitsof the converted digital data, a capacitative element 3003 to input theramp voltage Vrf to the comparator circuit 3005 as a reference voltageat the time of the execution of the second conversion process to findthe unconverted bits after the first conversion process, a capacitativeelement 3004 and the capacitative element 3003 to hold the voltage valueof the reference voltage at the time of the output change of thecomparator circuit 3005 in the first conversion process, and a logiccircuit 3006 to generate a control signal to hold the ramp voltage Vrcin the capacitative element 3004 at the time of the output change of thecomparator circuit 3005 in the first conversion process.

According to the column-parallel A/D converter circuit disclosed in thepatent document 3, in the first conversion process, the conversionobject analog voltage signal Vpix is inputted to the comparator circuit3005 through the S/H circuit 3001, and the switch circuit 3002 is turnedon and the ramp voltage Vrc is inputted. In addition, a configuration ofthe ramp voltage Vrc is the same as that of the ramp voltage Vrc shownin FIGS. 8 and 9. When the output of the comparator circuit 3005 isinverted while the ramp voltage Vrc monotonously increases step-by-step,the switch circuit 3002 is turned off by the control signal from thelogic circuit 3006, and the ramp voltage Vrc is held in the capacitativeelement 3004. At this time, the digital value of the counter circuit isstored in a memory circuit (not shown) as the value of the upper-orderbits of the digital data. In the second conversion process, the rampvoltage Vrf monotonously decreasing step-by-step is inputted. When theoutput of the comparator circuit 3005 is inverted, the digital value ofthe counter circuit is stored in the memory circuit (not shown) as thevalue of the lower-order bits of the digital data. In addition, aconfiguration of the voltage Vrf is the same as that of the ramp voltageVrf shown in FIGS. 8 and 9.

As shown in FIG. 14, since the column-parallel A/D converter circuitdisclosed in the patent document 3 is composed of the one S/H circuit,the two capacitative elements, and the switch circuit, its circuit sizecan be small.

As still another sub-ranging column-parallel A/D converter circuit,there is a column-parallel A/D converter circuit internally containingan integration circuit (refer to a patent document 4, for example), asshown in FIG. 15, which is composed of an input circuit 4010 to receivethe conversion object analog voltage signal Vpix and reference voltagesVDE1 and VDE2, an integration circuit 4020 to integrate the referencevoltage VDE 1 outputted from the input circuit 4010 at the time of afirst conversion process, and the reference voltage VDE2 outputted fromthe input circuit 4010 at the time of a second conversion process, acomparator circuit 4030 to compare an output voltage of the integrationcircuit 4020 with a reference voltage Vref, and a logic circuit 4040 togenerate a control signal used to hold the voltage level of theintegration circuit 4020 at the time of output change of the comparatorcircuit 4030 in the first conversion process. In addition, FIG. 16 showsa variation of the column-parallel A/D converter circuit internallycontaining the integration circuit disclosed in the patent document 4,and its operation principle is the same as that of the column-parallelA/D converter circuit internally containing the integration circuitshown in FIG. 15.

According to the column-parallel A/D converter circuit disclosed in thepatent document 4, the integration circuit 4020 integrates the referencevoltage VDE1 (corresponding to the first change amount ΔV1 in the patentdocument 2 and the patent document 3) and the reference voltage VDE2(corresponding to the second change amount ΔV2 in the patent document 2and the patent document 3) to generate the voltage corresponding to theupper-order bits and the voltage corresponding to the lower-order bitsto be compared with the conversion object analog voltage signal Vpix,without using the monotonously increasing ramp voltage Vrc and themonotonously decreasing ramp voltage Vrf unlike in the patent document 2and the patent document 3.

According to the column-parallel A/D converter circuit disclosed in thepatent document 4, before the execution of the first conversion process,the switch circuit 4012 is turned on and the switch circuit 4014 isturned off and then the conversion object analog voltage signal Vpix isinputted to the integration circuit 4020. In the first conversionprocess, the switch circuit 4012 is turned off and the switch circuit4013 is connected to the side of the reference voltage VDE1 and then theswitch circuit 4014 is turned on, and then the reference voltage VDE1(corresponding to the first ramp voltage Vrc having a relatively largeinclination) is integrated. When the output value of the comparatorcircuit 4030 is inverted, the digital value of the counter circuit (notshown) at this time is stored as the value of the upper-order bits, andthe switch circuit 4014 is turned off by the control signal outputtedfrom the logic circuit 4040. Thus, the difference voltage value Vdif isheld in the integration circuit 4020. In the second conversion process,the switch circuit 4013 is connected to the side of the referencevoltage VDE2 and then the switch circuit 4014 is turned on, and then thereference voltage VDE2 (corresponding to the second ramp voltage Vrfhaving an inclination smaller than that of the first ramp voltage Vrc)is integrated. When the output value of the comparator circuit 4030 isinverted, the digital value of the counter circuit (not shown) at thistime is stored as the value of the lower-order bits, and the secondconversion process is completed.

As described above, since the conversion object analog voltage signal isconverted to the digital data by the two conversion processes in thesub-ranging column-parallel A/D converter circuit disclosed in thepatent documents 2 to 4, the number of steps for the conversion processcan be considerably reduced, so that the time required for theconversion process can be shortened.

Patent document 1 Japanese Unexamined Patent Publication No. 2000-286706

Patent document 2 Japanese Unexamined Patent Publication No. 1999-168383

Patent document 3 Japanese Unexamined Patent Publication No. 2002-232291

Patent document 4 Japanese Unexamined Patent Publication No. 2005-348325

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, according to the column-parallel A/D converter circuitdisclosed in the patent document 2, the circuit size of the conversioncircuit is large due to the two comparator circuits each having arelatively large circuit area. In addition, according to thecolumn-parallel A/D converter circuit disclosed in the patent document4, a circuit size of the conversion circuit is large due to theintegration circuit having a relatively large circuit area. Especially,in the case of the column-parallel A/D converter circuit mounted on thesolid-state imaging element, the number of pixels (number of solid-stateimaging elements) considerably increases because the resolution of thesolid-state imaging element becomes high recently, and accordingly thenumber of conversion circuits used in the solid-state imaging deviceconsiderably increases, and the circuit size problematically becomeslarge. Furthermore, recently, a unit area of the solid-state imagingelement constituting the solid-state imaging device is minimized, and apixel pitch is narrowed, so that the A/D converter circuit is requiredto minimize its circuit size so as to be arranged in conjunction withthe small pixel pitch.

Meanwhile, according to the column-parallel A/D converter circuitdisclosed in the patent document 3, since the one S/H circuit and thetwo capacitative elements having relatively small area are used, thecircuit size of the conversion circuit can be small. However, accordingto the column-parallel A/D converter circuit disclosed in the patentdocument 3, when there is a variation in characteristics (capacityratios) of the two capacitative elements, the problem is that it ishighly likely that a variation is generated between the first rampvoltage Vrc inputted to the comparator 3005 without passing through thecapacitative element and the second ramp voltage Vrf inputted to thecomparator 3005 through the capacitative element. When thecolumn-parallel A/D converter circuit is mounted on the solid-stateimaging device, the variation between the first ramp voltage Vrc and thesecond ramp voltage Vrf becomes a noise, which could degrade imagequality. More specifically, it appears as a variation between columns onvertical lines on an image taken by the solid-state imaging element. Inaddition, when the capacities of the two capacitative elements areincreased to suppress the variation in capacity ratio with a view topreventing the image quality from degrading, the circuit sizeproblematically increases.

In addition, according to the column-parallel A/D converter circuitdisclosed in the patent document 2, when there is a variation incharacteristics of the difference detection circuit and the attenuationcircuit, the problem is that a variation is sometimes generated betweenthe first ramp voltage Vrc used in the first conversion process to findthe upper-order bits, and the second ramp voltage Vrf used in the secondconversion process to find the lower-order bits. When thecolumn-parallel A/D converter circuit is mounted on the solid-stateimaging device, the variation between the first ramp voltage Vrc and thesecond ramp voltage Vrf becomes a noise, which could degrade imagequality. More specifically, it appears as a variation between columns onvertical lines on an image taken by the solid-state imaging element.

In addition, according to the column-parallel A/D converter circuitdisclosed in the patent document 4 shown in FIG. 15, since theconversion object analog voltage signal Vpix is directly inputted to anoutput node of the integration circuit 4020 through the S/H circuit 4011and the switch circuit 4012, it is affected by variations of a resistorelement 4021, an amplifier circuit 4022, and a capacitative element 4023in the integration circuit 4020, and the problem is that its imagequality could degrade. In addition, according to the column-parallel A/Dconverter circuit disclosed in the patent document 4 shown in FIG. 16,since the conversion object analog voltage signal Vpix is inputted to aninput node of the integration circuit 5020 through the S/H circuit 5011and the switch circuit 5013, variations of a resistor element 5021, anamplifier circuit 5022, and a capacitative element 5023 in theintegration circuit 5020 can be effectively cancelled. However, in thecase shown in FIG. 16, since a time to integrate the conversion objectanalog voltage signal Vpix in the integration circuit 5020 is newlyneeded, the problem is that its operation speed cannot be high enough.

Recently, the solid-state imaging device is required to improve theconversion process speed of the A/D converter circuit, to prevent imagequality from degrading due to a characteristic variation (to preventconversion accuracy from degrading), and to reduce a circuit size.However, as described above, both of prevention of degradation inconversion accuracy and prevention of increase in circuit size cannot beimplemented by any one of the column-parallel A/D converter circuitsdisclosed in the patent document 2 to the patent document 4.

The present invention was made in view of the above problems, and it isan object of the present invention to provide a sub-ranging A/Dconverter circuit capable of effectively implementing both of theprevention of degradation in conversion accuracy due to thecharacteristic variation and prevention of increase in circuit size.Furthermore, it is an object of the present invention to provide asolid-state imaging device provided with a sub-ranging A/D convertercircuit capable of effectively implementing both of the prevention ofdegradation in conversion accuracy due to the characteristic variationand prevention of increase in circuit size.

Means for Solving the Problem

As first characteristics, an A/D converter circuit according to thepresent invention in order to attain the above object comprises: a firstcapacitative element; a second capacitative element having an output endconnected to an input end of the first capacitative element; a thirdcapacitative element having an output end connected to an input end ofthe second capacitative element; a voltage comparator circuit having aninput terminal connected to an output end of the first capacitativeelement, configured to compare a voltage value of the input terminalwith a predetermined threshold voltage value; a first input circuitconfigured to input an externally inputted conversion object analogvoltage signal to a first intermediate node provided between the firstcapacitative element and the second capacitative element; a second inputcircuit configured to input a first reference voltage to a secondintermediate node provided between the second capacitative element andthe third capacitative element, the first reference voltage being usedfor a first conversion process to find a value of sequential bitscontaining the most significant bit, in digital data provided after theconversion object analog voltage signal has been converted; a thirdinput circuit configured to input a second reference voltage to an inputend of the third cap acitative element, the second reference voltagebeing used for a second conversion process to find a value ofunconverted bits in the digital data after the first conversion process;a control circuit configured to generate a control signal to hold avoltage value of the first reference voltage in the third cap acitativeelement when an output of the voltage comparator circuit changes in thefirst conversion process; a first ramp voltage generator circuitconfigured to generate a first ramp voltage having a voltage valuemonotonously changing step-by-step, and output the first ramp voltage asthe first reference voltage while at least the first conversion processis executed; a second ramp voltage generator circuit configured togenerate a second ramp voltage having a voltage value monotonouslychanging step-by-step or sequentially, and output the second rampvoltage as the second reference voltage while at least the secondconversion process is executed; a counter circuit configured to output adigital value counted in response to a change of the first ramp voltageand a change of the second ramp voltage; and a memory circuit configuredto store the digital value when the output of the voltage comparatorcircuit changes.

As second characteristics, the A/D converter circuit according to thepresent invention is further configured in such a manner that the firstinput circuit includes a first switch circuit configured to set whetheror not the conversion object analog voltage signal is inputted to thefirst intermediate node, and turns on the first switch circuit in apredetermined period before the counter circuit starts counting in thefirst conversion process, the second input circuit includes a secondswitch circuit configured to set whether or not the first referencevoltage is inputted to the second intermediate node, and turns on thesecond switch circuit in the first conversion process, and the thirdinput circuit includes a third switch circuit configured to set whetheror not the second reference voltage is inputted to the thirdcapacitative element, and turns off the third switch circuit in a periodfrom when the output of the voltage comparator circuit changes to whenthe second conversion process starts, based on the control signal.

As third characteristics, the A/D converter circuit according to thepresent invention is further configured in such a manner that the firstinput circuit includes a first switch circuit configured to set whetheror not the conversion object analog voltage signal is inputted to thefirst intermediate node, and turns on the first switch circuit in apredetermined period before the counter circuit starts counting in thefirst conversion process, the second input circuit includes a fourthswitch circuit configured to divide the second intermediate node into afirst divided node on the side of the second capacitative element and asecond divided node on the side of the third cap acitative element inorder to control an electric connection between the second capacitativeelement and the third capacitative element, a second switch circuitconfigured to set whether or not the first reference voltage is inputtedto the first divided node, and a fifth switch circuit configured to setwhether or not a third reference voltage such as the first referencevoltage or a voltage corresponding to the first reference voltage isinputted to the second divided node, and is configured to turn on thesecond switch circuit in the first conversion process, turn off thefourth switch circuit in the first conversion process, and turn on thefifth switch circuit in a period from when the counter circuit startscounting to when the output of the voltage comparator circuit changes inthe first conversion process, based on the control signal, to hold thevoltage value of the first reference voltage when the output of thevoltage comparator circuit changes, in the third capacitative element,and the third input circuit is configured to directly input the secondreference voltage to an input terminal of the third capacitativeelement.

As fourth characteristics, the A/D converter circuit according to thepresent invention is further configured in such a manner that the secondramp voltage generator circuit generates the second ramp voltagechanging in an opposite direction to the change in the first rampvoltage in the first conversion process, in the second conversionprocess.

As fifth characteristics, the A/D converter circuit according to thepresent invention is further configured in such a manner that thevoltage comparator circuit includes an inverter circuit, and a sixthswitch circuit configured to cause short circuit between an inputterminal and an output terminal of the inverter circuit, and shortcircuit is caused in the inverter circuit by the sixth switch circuitfor initialization in a predetermined initialization period before theconversion object analog voltage signal is inputted to the firstintermediate node by the first input circuit.

As sixth characteristics, the A/D converter circuit according to thepresent invention is further configured in such a manner that thecontrol circuit controls an output timing of the control signal so asnot to switch a switch circuit controlled by the control signal fromwhen the first ramp voltage inputted to the third capacitative elementchanges to when a voltage value changing in response to a change in thefirst ramp voltage becomes stable at the input end and the output end ofthe third capacitative element, in the first conversion process.

As seventh characteristics, the A/D converter circuit according to thepresent invention is further configured in such a manner that the firstramp voltage monotonously changes step-by-step by a first change amountprovided by dividing a voltage width of the conversion object analogvoltage signal by a split number based on a first resolution previouslyset according to a bit number of the digital data, and the second rampvoltage monotonously changes step-by-step by a second change amountprovided by dividing the first change amount by a split number based ona second resolution defined so that a sum of a value of a bit numbercorresponding to the second resolution and a value of a bit numbercorresponding to the first resolution is equal to a value of the bitnumber of the digital data.

As eighth characteristics, the A/D converter circuit according to thepresent invention is further configured in such a manner that the firstramp voltage monotonously changes step-by-step by a first change amountprovided by dividing a voltage width of the conversion object analogvoltage signal by a split number based on a first resolution previouslyset according to a bit number of the digital data provided after theconversion object analog voltage signal has been converted, and thesecond ramp voltage sequentially and monotonously changes.

As ninth characteristics, the A/D converter circuit according to thepresent invention is further configured in such a manner that the secondramp voltage generator circuit generates and outputs the second rampvoltage in a period including one or both of a period just before thesecond conversion process and a period just after the second conversionprocess, in addition to an execution period of the second conversionprocess.

A solid-state imaging device according to the present invention toattain the above object includes: a solid-state imaging element groupcomposed of a plurality of solid-state imaging elements arranged in amatrix and configured to convert an optical signal to an electricsignal; a first capacitative element, a second capacitative element, athird capacitative element, a voltage comparator circuit, a first inputcircuit, a second input circuit, a third input circuit, a controlcircuit, and memory circuits provided with respect to each column of thesolid-state imaging element group; and a first ramp voltage generatorcircuit, a second ramp voltage generator circuit, and a counter circuitprovided on the same chip with respect to the solid-state imagingelement group, wherein the first capacitative element, the secondcapacitative element, the third capacitative element, the voltagecomparator circuit, the first input circuit, the second input circuit,the third input circuit, the control circuit, the memory circuits, thefirst ramp voltage generator circuit, the second ramp voltage generatorcircuit, and the counter circuit constitute the A/D converter circuithaving the above first to ninth characteristics.

EFFECT OF THE INVENTION

According to the above A/D converter circuit, since the sub-ranging A/Dconverter circuit is composed of the three capacitative elements havingrelatively small circuit areas, and the voltage comparator circuitserving as a logic circuit such as an inverter circuit having arelatively small area, increase in circuit area can be suppressed. Inaddition, the above A/D converter uses the first capacitative element,the second capacitative element, and the third capacitative element tohold the difference voltage, using the auto-zero technique to cancel thecharacteristic variation in inversion level of the voltage comparatorcircuit, so that the variation in absolute values of the capacity valuesof the first capacitative element, the second capacitative element, andthe third capacitative element do not directly affect the A/D convertedresult. Therefore, the conversion accuracy can be effectively preventedfrom degrading due to the variation among the circuits. Since thedegradation in conversion accuracy can be effectively prevented in thepresent invention, the noise in image quality is more effectivelyprevented from being generated in the taken image when the presentinvention is applied to a solid-state imaging device. That is, accordingto the above A/D converter circuit, both of the prevention ofdegradation in conversion accuracy and the prevention of increase incircuit size can be effectively implemented.

In addition, according to the A/D converter circuit having the fourthcharacteristics, since the second ramp voltage changing in the oppositedirection to the change in the first ramp voltage is generated, thethird capacitative element after the first conversion process can bedirectly used in the second conversion process, so that the A/Dconverter circuit according to the present invention can be implementedwith the relatively simple circuit configuration.

According to the A/D converter circuit having the fifth characteristics,since the voltage comparator circuit is composed of the inverter circuitand the switch circuit each having a relatively small area, the voltagecomparator circuit can be implemented with the simple configuration, andthe increase in circuit size can be prevented.

According to the A/D converter circuit having the sixth characteristics,since the state of the switch circuit is controlled by the controlsignal so as not be changed from when the first ramp voltage changes towhen the voltages at the input end and the output end of the thirdcapacitative element becomes stable in the first conversion process, thevoltage at the input end or the output end of the third capacitativeelement can be sufficiently settled down in the first conversionprocess, and the voltage can be held in the third capacitative elementunder the condition that the voltage value of the first ramp voltage isstable. Thus, the variation caused in the circuit can be effectivelyprevented. In addition, when the A/D converter circuit having the sixthcharacteristics is used in the solid-state imaging device, the imagequality can be effectively prevented from degrading due to the noise.

According to the A/D converter circuit having the eighthcharacteristics, since the second ramp voltage is configured tosequentially and monotonously changes, in the case of the first processto hold the first ramp voltage in the third cap acitative element at thetime of the output change of the voltage comparator circuit, the firstramp voltage keeping the same voltage value stably in the given periodis used, and in the case of the second conversion process in which thesecond ramp voltage is not held, the second ramp voltage superior inmonotonous increasing property is used. Thus, the frequency of thecounter circuit can be improved in the second conversion process, sothat the bit resolution can be highly accurate by a relatively easy way.

According to the A/D converter circuit having the ninth characteristics,since the second ramp voltage generator circuit is configured togenerate the second ramp voltage in the period including one or both ofthe given period just before the second conversion process and the givenperiod just after the second conversion process, in addition to theexecution period of the second conversion process, it can moreeffectively solve the problem that the offset voltage is generated inthe difference voltage and the voltage comparator circuit cannot detectthe difference voltage in the second process because the differencebetween the difference voltage of the conversion object analog voltagesignal used in the second conversion process, and the first ramp voltageat the time of the output change of the voltage comparator circuit inthe first conversion process is very small, or because the value of thedifference voltage is roughly equal to the value of the first changeamount.

According to the solid-state imaging device having the abovecharacteristics, since it is provided with the A/D converter circuithaving the first to tenth characteristics, the increase in circuit areaof the A/D converter circuit can be prevented, the noise is effectivelyprevented from being generated due to the variation in characteristicsof the transistor element and the capacitative element, and theconversion accuracy can be effectively prevented from degrading due tothe generation of the noise, while using the sub-ranging A/D convertercircuit capable of implementing a relatively high frame rate.Furthermore, according to the solid-state imaging device having theabove characteristics, since the noise can be effectively prevented frombeing generated in the A/D converter circuit, degradation in the imagequality of the taken image, such as noise generated in the form of avertical line in the taken image, can be prevented more effectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic partial block diagram showing a schematicconfiguration example in a first embodiment of a solid-state imagingdevice according to the present invention.

FIG. 2 is a schematic circuit diagram showing a schematic configurationexample in the first embodiment of an A/D converter circuit according tothe present invention.

FIG. 3 is a schematic waveform diagram showing a waveform of eachterminal in the first embodiment of the A/D converter circuit accordingto the present invention.

FIG. 4 is a schematic partial block diagram showing a schematicconfiguration example in the first embodiment of a solid-state imagingdevice according to the present invention.

FIG. 5 is a schematic circuit diagram showing a schematic configurationexample in a second embodiment of an A/D converter circuit according tothe present invention.

FIG. 6 is a schematic waveform diagram showing a waveform of eachterminal in the second embodiment of the A/D converter circuit accordingto the present invention.

FIG. 7 is a schematic waveform diagram showing a schematic voltagewaveform example of a second ramp voltage in another embodiment of anA/D converter circuit according to the present invention.

FIG. 8 is an explanatory view showing a relationship of each voltage ofa general sub-ranging column-parallel A/D converter circuit.

FIG. 9 is a waveform diagram showing a schematic configuration exampleof a ramp voltage used in the general sub-ranging column-parallel A/Dconverter circuit.

FIG. 10 is a schematic partial circuit diagram showing a schematicconfiguration example of a solid-state imaging device according to aconventional technique.

FIG. 11 is a waveform diagram showing signal waveforms and an internalstate in a column-parallel A/D converter circuit according to theconventional technique.

FIG. 12 is a waveform diagram showing a schematic configuration exampleof a ramp voltage used in the column-parallel A/D converter circuitaccording to the conventional technique.

FIG. 13 is a schematic partial circuit diagram showing a schematicconfiguration example of a sub-ranging column-parallel A/D convertercircuit internally containing a difference detection circuit accordingto a conventional technique.

FIG. 14 is a schematic partial circuit diagram showing a schematicconfiguration example of a sub-ranging column-parallel A/D convertercircuit using a capacity ratio according to a conventional technique.

FIG. 15 is a schematic partial circuit diagram showing a schematicconfiguration example of a sub-ranging column-parallel A/D convertercircuit internally containing an integration circuit according to aconventional technique.

FIG. 16 is a schematic partial circuit diagram showing a schematicconfiguration example of a sub-ranging column-parallel A/D convertercircuit internally containing an integration circuit according to aconventional technique.

EXPLANATION OF REFERENCES

-   -   1 Solid-state Imaging Device according to the present invention    -   1A Solid-state Imaging Device according to the present invention    -   1B Solid-state Imaging Device according to the present invention    -   10 A/D Converter Unit    -   12 Control Circuit    -   20 Ramp Voltage Generator Circuit    -   21 First Ramp Voltage Generator Circuit    -   22 Second Ramp Voltage Generator Circuit    -   23 First Ramp Voltage Generator Circuit    -   30 Control Signal Generator Circuit    -   40 Counter Circuit    -   50 Memory Circuit    -   51 Upper-order Bit Memory Region    -   52 Lower-order Bit Memory Region    -   60 A/D Converter Unit    -   62 Control Circuit    -   100 A/D Converter Circuit according to the present invention    -   200 A/D Converter Circuit according to the present invention    -   121 D Flip-Flop Circuit    -   122 Negative AND Circuit    -   123 Inverter Circuit    -   621 D Flip-Flop Circuit    -   622 Negative AND Circuit    -   IPD Solid-state Imaging Element Group    -   PIX Solid-state Imaging Element    -   HD Horizontal Decoder    -   VD Vertical Decoder    -   C1 First Capacitative Element    -   C2 Second Capacitative Element    -   C3 Third Capacitative Element    -   N1 First Intermediate Node    -   N2 Second Intermediate Node    -   N3 Third Intermediate Node    -   ND1 First Divided Node    -   ND2 Second Divided Node    -   CMP Voltage Comparator Circuit    -   SW1 First Switch Circuit    -   SW2 Second Switch Circuit    -   SW3 Third Switch Circuit    -   SW4 Fourth Switch Circuit    -   SW5 Fifth Switch Circuit    -   SW6 Sixth Switch Circuit

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a description will be made of embodiments of an A/Dconverter circuit and a solid-state imaging device according to thepresent invention (hereinafter, referred to as the “circuit of thepresent invention” and the “device of the present invention”occasionally) with reference to the drawings.

First Embodiment

A first embodiment of the circuit of the present invention and thedevice of the present invention will be described with reference toFIGS. 1 to 3.

First, configurations of the circuit of the present invention and thedevice of the present invention will be described with reference toFIGS. 1 and 2. Here, FIG. 1 shows a schematic configuration example of adevice 1A of the present invention, on which a circuit 100A of thepresent invention is mounted, and FIG. 2 shows a schematic configurationexample of the circuit 100A of the present invention according to thisembodiment.

In addition, the circuit 100A of the present invention is a sub-rangingA/D converter circuit to execute a two-step conversion process composedof a first conversion process to find a value of sequential partial bitscontaining the most significant bit, in digital data provided after aconversion object analog voltage signal Vpix has been converted, and asecond conversion process to find a value of unconverted bits in thedigital data after the first conversion process. In addition, accordingto this embodiment, in order to simplify the description, it is assumedthat the digital data is composed of 4 bits (=2⁴) provided after theconversion object analog voltage signal Vpix has been converted, and theupper-order 2 bits are converted in the first conversion process, andthe lower-order 2 bits are converted in the second conversion process.While a method for dividing into the upper-order bits and thelower-order bits is not limited, it is preferable to set the number ofupper-order bits and the number of lower-order bits in such a mannerthat their difference is minimal with a view to shortening a conversionprocess time.

As shown in FIG. 1, the device 1A of the present invention is composedof, on the same chip, a solid-state imaging element group IPD in which aplurality of solid-state imaging elements PIX_(ij) (i=1 to m, j=1 to n)each to convert an optical signal to an analog voltage signal Vpix arearranged in the form of a matrix, a plurality of A/D converter units 10each provided with respect to each column of the solid-state imagingelement group IPD, a memory circuit 50 in which a memory is providedwith respect to each column of the solid-state imaging element groupIPD, and those provided with respect to the solid-state imaging elementgroup IPD, such as a first ramp voltage generator circuit 21 to generatea first ramp voltage Vrc used in the first conversion process, a secondramp voltage generator circuit 22 to generate a second ramp voltage Vrfused in the second conversion process, a control signal generatorcircuit 30 to generate various clock signals and control signals used inthe A/D converter unit 10, a counter circuit 40 to output a counteddigital value in response to a voltage change of the first ramp voltageVrc and a voltage change of the second ramp voltage Vrf, a verticaldecoder VD to select a reading target row in a reading process, and ahorizontal decoder HD to select a reading target column in the readingprocess.

More specifically, as shown in FIGS. 1 and 2, the circuit 100A of thepresent invention is composed of the A/D converter units 10, the memorycircuit 50, the first ramp voltage generator circuit 21, the second rampvoltage generator circuit 22, the control signal generator circuit 30,and the counter circuit 40.

As shown in FIG. 2, the A/D converter unit 10 is composed of a firstcapacitative element C1, a second capacitative element C2 having anoutput end connected to an input end of the first capacitative elementC1, a third capacitative element C3 having an output end connected to aninput end of the second capacitative element C2, a voltage comparatorcircuit CMP having an input terminal connected to an output end of thefirst capacitative element C1 to compare a voltage value of the inputterminal with a predetermined threshold voltage value Vth, a first inputcircuit to input the externally inputted conversion object analogvoltage signal Vpix to a first intermediate node N1 between the firstcapacitative element C1 and the second capacitative element C2, a secondinput circuit to input the first reference voltage used for the firstconversion process to a second intermediate node N2 between the secondcapacitative element C2 and the third capacitative element C3, a thirdinput circuit to input the second reference voltage used for the secondconversion process to an input end of the third capacitative element C3,and a control circuit 12 to generate a control signal Vctl to hold avoltage value of the first reference voltage in the third capacitativeelement C3 at the time of output change of the voltage comparatorcircuit CMP in the first conversion process.

More specifically, the first input circuit is provided with a switchcircuit SW1 to set whether or not the conversion object analog voltagesignal Vpix can be inputted to the first intermediate node N1, and it isconfigured to turn on the switch circuit SW1 during a given periodbefore the counter circuit 40 starts counting in the first conversionprocess.

The second input circuit is provided with a switch circuit SW2 to setwhether or not the first reference voltage can be inputted to the secondintermediate node N2, and it is configured to turn on the switch circuitSW2 at the time of the first conversion process.

The third input circuit is provided with a switch circuit SW3 to setwhether or not the second reference voltage can be inputted to the thirdcapacitative element C3, and the switch circuit SW3 is connected to theinput end of the third capacitative element C3 through the thirdintermediate node N3. The third input circuit is configured to keep theswitch circuit SW3 in off state during a period from the output changeof the voltage comparator circuit CMP to the start of the secondconversion process, based on the control signal Vctl.

The voltage comparator circuit CMP is provided with an inverter circuitand a switch circuit SW6 to cause short circuit between an inputterminal and an output terminal of the inverter circuit, and the switchcircuit SW6 causes short circuit in the inverter circuit forinitialization during a given initializing period before the conversionobject analog voltage signal Vpix is inputted to the first intermediatenode N1 by the first input circuit.

As shown in FIG. 2, the control circuit 12 is composed of a negative ANDcircuit 122 to calculate a negative AND between an inversion signal ofan output signal Vcp of the voltage comparator circuit CMP outputtedfrom a negative AND circuit 123, and an external input signal C_PHshowing that the first conversion process is being executed, and a Dflip-flop circuit (DFF circuit) 121 to latch and output an output signalfrom the negative AND circuit 122 when an externally inputted clocksignal SCLK rises. In addition, according to this embodiment, ascompared with a clock signal CLK used to generate the first ramp voltageVrc by the first voltage generator circuit 21 to be described below, theclock signal SCLK has the same frequency but its phase is set to beshifted behind 3/4 cycle in view of a settling time of voltages of theinput end and the output end of the capacitative element C3. Thus, inthe first conversion process, the switch circuit SW3 can be switchedwhile the first ramp voltage Vrc is stable.

The first ramp voltage generator circuit 21 generates the first rampvoltage Vrc whose voltage value monotonously changes step-by-step insynchronization with the clock signal CLK, during at least an executiontime of the first conversion process, and outputs it as the firstreference voltage.

More specifically, the first ramp voltage Vrc is configured tomonotonously change step-by-step by a first change amount AV1 providedby dividing a voltage width of the conversion object analog voltagesignal Vpix by a split number based on a first resolution R1 which hasbeen previously set based on the bit number of the digital data.

More specifically, since the upper-order 2 bits are converted in thefirst conversion process according to this embodiment, the firstresolution R1 is set to 2 bits. In this case, the split number based onthe first resolution R1 is such that 2^(R1)=2²=4, and the voltage valueprovided by dividing the voltage width of the conversion object analogvoltage signal Vpix by 4 corresponds to the first change amount ΔV1(that is, the first change amount ΔV1×4 is equal to the voltage width ofthe conversion object analog voltage signal Vpix).

In addition, according to this embodiment, as will be shown in FIG. 3,since it is assumed that a voltage value of the conversion object analogvoltage signal Vpix at the time of loading an image is lower than areset level of the conversion object analog voltage signal Vpix, thedescription is made assuming that the first ramp voltage Vrcmonotonously increases step-by-step, while it may monotonously decreasestep-by-step depending on configurations of the conversion object analogvoltage signal Vpix and another circuit.

The second ramp voltage generator circuit 22 generates the second rampvoltage Vrf whose voltage value monotonously changes step-by-step insynchronization with the clock signal CLK during at least an executiontime of the second conversion process, and outputs it as the secondreference voltage. More specifically, the second ramp voltage Vrfmonotonously changes step-by-step by a second change amount AV2 providedby dividing the first change amount AV1 by split number based on asecond resolution R2 which is defined in such a manner that a sum of thebit number corresponding to the second resolution R2 and the bit numbercorresponding to the first resolution R1 becomes the bit number of thedigital data.

More specifically, the second resolution R2 is set such that the digitaldata bit number=the first resolution bit number R1+second resolution bitnumber R2. As described above, according to this embodiment, since thedigital data bit number is set to 4, and the first resolution R1 is setto 2 bits, the second resolution R2 is 2 bits. Therefore, the splitnumber based on the second resolution R2 is such that 2^(R2)=2²=4, andthe second change amount ΔV2=the first change amount ΔV1/4.

In addition, according to this embodiment, the second ramp voltage Vrfin the second conversion process changes in the opposite direction tothe change in the first ramp voltage Vrc in the first conversionprocess. That is, according to this embodiment, the second ramp voltageVrf is configured to monotonously decrease step-by-step in the secondconversion process as will be shown in FIG. 3. However, it may beconfigured to monotonously increase when the first ramp voltage Vrc isconfigured to monotonously decrease or depending on a circuitconfiguration.

The control signal generator circuit 30 generates various kinds of clocksignals and control signals used in the A/D converter unit 10, such asthe clock signal CLK, the clock signal SCLK, and the signal C_PH showingthat the first conversion process is being executed as shown in FIG. 3.Furthermore, the control signal generator circuit 30 generates controlsignals to control the switching between on states and off states of theswitch circuits SW6, SW3, and SW2.

The memory circuit 50 is composed of a upper-order bit memory region 51and a lower-order bit memory region 52 with respect to each column ofthe solid-state imaging element group IPD, and when an output of the A/Dconverter unit 10 changes in the first conversion process, a digitalvalue outputted from the counter circuit 40 is stored in the upper-orderbit memory region 51, and when the output of the A/D converter unit 10changes in the second conversion process, a digital value outputted fromthe counter circuit 40 is stored in the lower-order bit memory region52.

More specifically, according to this embodiment, when an output level ofthe voltage comparator circuit CMP shifts from H level to L level, adigital value Cu outputted from the counter circuit 40 is stored in theupper-order bit memory region 51, and when the output level of thevoltage comparator circuit CMP shifts from L level to H level, a digitalvalue Cl outputted from the counter circuit 40 is stored in thelower-order bit memory region 52. In this configuration, when thecounter values are stored in the upper-order bit memory region 51 andthe lower-order bit memory region 52 of the memory circuit 50, thecomplicated control circuit 12 is not needed, so that the circuitconfiguration can be simple.

Next, a description will be made of operations of the circuit 100A ofthe present invention and the device 1A of the present invention, withreference to FIG. 3. Here, FIG. 3 shows signal waveforms and an internalstate of the circuit 100A of the present invention according to thisembodiment.

When an imaging process starts at a time t0, the device 1A of thepresent invention starts an initialization process to initialize eachcircuit in the circuit 100A of the present invention at a time t1. Inthe initialization process, a voltage Vrst of the solid-state imagingelement PIX_(ij) at a reset level is sampled.

More specifically, as shown in FIG. 3, at the time t1, the voltage valueof the conversion object analog voltage signal Vpix outputted from thesolid-state imaging element PIX_(ij) is at the reset level Vrst, thevoltage value of the first ramp voltage Vrc is at an initial level Vrc0,and the voltage value of the second ramp voltage Vrf is at an initiallevel Vrf0. In addition, the switch circuits SW1, SW2, and SW6 are inoff state, and the switch circuit SW3 is in on state.

When the initialization process is started at the time t1, the switchcircuit SW6 is turned on to cause short circuit between the inputterminal and the output terminal of the voltage comparator circuit CMP,and a voltage of the voltage comparator circuit CMP at an inversionlevel Vth is inputted to the output end of the capacitative element C1.In addition, when the switch circuit SW1 is turned on at the time t1,the voltage signal at the reset level Vrst outputted from thesolid-state imaging element PIX in the reading target row selected bythe vertical decoder VD is inputted to the input end (first intermediatenode N1) of the capacitative element C1.

In addition, as shown in FIG. 3, since the switch circuit SW3 is in onstate, the voltage level of the third intermediate node N3 at this timeis at the same voltage level as that of the second ramp voltage Vrfinputted through the switch circuit SW3, that is, the initial levelVrf0.

Then, at a time t2, when the switch circuit SW6 is turned off and shortcircuit of the voltage comparator circuit CMP is canceled, a differencevoltage between the reset level Vrst and the inversion level Vth of thevoltage comparator circuit CMP is held in the capacitative element C1,and the initialization process is completed. Then, by the imagingoperation of the solid-state imaging element PIX, the conversion objectanalog voltage signal Vpix at a pixel level Vsig (voltage Vsig<voltageVrst in the solid-state imaging element PIX according to thisembodiment) is outputted from the solid-state imaging element PIX basedon light intensity of an imaging target. Here, a difference voltage Va(absolute value of (Vsig−Vrst)) between the pixel level Vsig and thereset level Vrst of the conversion object analog voltage signal Vpix isthe target of the first conversion process.

At a time t3, a sampling process to sample the signal level Vsig of thesolid-state imaging element PIX_(ij) is started in the image loadingprocess.

More specifically, at the time t3, the switch circuit SW2 is turned on,while the switch circuit SW1 is kept in on state. Thus, the first rampvoltage Vrc at the initial level Vrc0 is inputted to the input end(second intermediate node N2) of the capacitative element C2, and theconversion object analog voltage signal Vpix at the pixel level Vsig isinputted to the output end (first intermediate node N1) of thecapacitative element C2.

At a time t4, when the switch circuit SW1 is turned off, the differencevoltage between the voltage value (initial level Vrc0) of the first rampvoltage Vrc and the voltage value (pixel level Vsig) of the conversionobject analog voltage signal Vpix at the time t4 is held in thecapacitative element C2, and the sampling process of the solid-stateimaging element PIX at the signal level Vsig is completed.

At a time t5, the external input signal C_PH showing that the firstconversion process is being executed becomes H level, and a sampling andholding process is started to hold the voltage used in the firstconversion process to find the upper-order bit value and the secondconversion process. In the first conversion process, the first rampvoltage generator circuit 21 generates the first ramp voltage Vrc whichmonotonously increases step-by-step by the first change amount ΔV1, insynchronization with the clock signal CLK of the device 1A of thepresent invention. The counter circuit 40 counts up from 0 by 1 insynchronization with the clock signal CLK in response to the increase ofthe ramp voltage, and outputs the digital value Cu [(x−1):0] (xrepresents the bit number of upper-order bits, or 2 in this embodiment).In addition, a voltage level Vrck (k=Cu) of the first ramp voltage Vrcis provided by adding the first change amount ΔT1×counter value Cu tothe initial level Vrc0 (Vrc0+ΔV1×Cu).

In addition, at this time, as shown in FIG. 2, the second intermediatenode N2 is at the initial level Vrc0 of the first ramp voltage Vrc. Inaddition, since the switch circuit SW2 is in on state at the time t5,the first ramp voltage Vrc is directly inputted to the secondintermediate node N2 after the time t5, so that as shown in FIG. 3, thevoltage level of the second intermediate node N2 rises every time thefirst ramp voltage Vrc rises. In addition, as shown in FIG. 2, since avoltage waveform of the second intermediate node N2 is gentle due to anon resistance of the switch circuit SW2 as compared with a voltagewaveform of the first ramp voltage Vrc, the on resistance of the switchcircuit SW2 is to be designed in view of a settling time of the voltageof the second intermediate node N2.

In addition, as shown in FIG. 2, the third intermediate node N3 isconfigured such that the first ramp voltage Vrc is inputted theretothrough the third capacitative element C3 and the switch circuit SW2,and as shown in FIG. 3, the voltage level thereof rises instantaneouslyevery time the first ramp voltage Vrc rises due to capacitive couplingof the third capacitative element C3. After that, the voltage level ofthe third intermediate node N3 returns to the original voltage levelVrf0, according to the on resistance of the switch circuit SW3 and a RCtime constant of the third capacitative element C3. In addition, thecapacity of the third capacitative element C3 is set based on a randomvariation due to kT/C noise (k: Boltzmann constant, T: absolutetemperature, C: capacity value) as a thermal noise, a resolution (imagequality accuracy) required for the circuit 100A of the presentinvention, and the voltage width (full-range, first change amount ΔV1×4)of the conversion object analog voltage signal Vpix. Furthermore, the onresistance of the switch circuit SW3 is to be designed in view of thecapacity of the third capacitative element C3, and the settling time ofthe voltage level of the third intermediate node N3 from itsinstantaneous rise to the original voltage level Vrf0.

At a time t6, when the voltage level Vrck (k=Cu) of the first rampvoltage Vrc becomes higher than the value provided by adding thedifference voltage Va to the initial voltage level Vrc0 of the firstramp voltage Vrc, that is, when the voltage level of the output end ofthe capacitative element C1 becomes higher than the inversion level Vthof the voltage comparator circuit CMP, the output level of the outputsignal Vcp of the voltage comparator circuit CMP is inverted from Hlevel to L level. The counter value Cu at this time is a conversionresult of the upper-order bits, and stored in the upper-order bit memoryregion 51. In addition, while only one A/D converter unit 10 is shown inFIG. 3, the time it takes for the level of the output signal Vcp of thevoltage comparator circuit CMP to be inverted differs according to thesolid-state imaging element PIX.

Here, in the following formulas 4 and 5, an input voltage Vin of thevoltage comparator circuit CMP in the circuit 100A of the presentinvention in the first conversion process is shown with the thresholdvoltage value Vth of the voltage comparator circuit CMP, the initiallevel Vrst of the solid-state imaging element PIX_(ij), the pixel levelVsig of the solid-state imaging element PIX_(ij) at the time of imagingoperation, and the initial level Vrc0 of the first ramp voltage Vrc.

Vin=Vrc+(Vsig−Vrc0)+(Vth−Vrst)  (4)

Vin−Vth=Vrc−{Vrc0+(Vrst−Vsig)}  (5)

Therefore, the first ramp voltage Vrc at the time of the output changeof the voltage comparator circuit CMP in the first conversion process isrepresented by the following formula 6, based on the formulas 4 and 5and FIG. 3. In addition, the first ramp voltage Vrc shown in the formula6 includes not only the voltage level in a normal period but also theinstantaneous voltage level at the time of rising.

Vrc=Vrc0+(Vrst−Vsig)=Vrc0+Va

(provided that Vin−Vth=0)  (6)

When the output level of the output signal Vcp of the voltage comparatorcircuit CMP is inverted, the control circuit 12 outputs the controlsignal Vctl to turn off the switch circuit SW3 at the time of rising (ata time t7) of the clock signal SCLK outputted from the control signalgenerator circuit 30. More specifically, before the output level of theoutput signal Vcp of the voltage comparator circuit CMP is inverted, theoutput signal Vcp of the voltage comparator circuit CMP is at H leveland the external input signal C_PH is at H level, so that the outputsignal of the negative AND circuit 122 is at H level. When the outputlevel of the output signal Vcp of the voltage comparator circuit CMP isinverted, the output signal of the voltage comparator circuit CMPbecomes L level, so that the output signal of the negative AND circuit122 becomes L level. Thus, at the time t7, the control signal Vctl toswitch the switch circuit SW3 from on state to off state is outputtedfrom the DFF circuit 121 at the time of rising of the clock signal SCLK.In addition, as for the clock signal SCLK, in order to prevent theswitch circuit SW3 from being switched at the time of the voltage changeof the first ramp voltage Vrc, its phase is set to be different fromthat of the clock signal CLK of the first ramp voltage generator circuit21, that is, it is shifted behind 3/4 cycle here, in view of thesettling time of the voltages at the input end and output end of thecapacitative element C3. Thus, the switch circuit SW3 can be switchedwhile the first ramp voltage Vrc is stable.

Here, as shown in FIG. 3, at the time t6, the initial level Vrf0 of thesecond ramp voltage Vrf is inputted to the input end of the capacitativeelement C3, and the voltage level Vrck of the first voltage Vrc isinputted to the output end of the capacitative element C3. At the timet7, when the switch circuit SW3 is turned off, the difference voltageVrck-Vrf0 between the voltage level Vrck of the first ramp voltage Vrcand the initial level Vrf0 of the second ramp voltage Vrf is held in thecapacitative element C3.

At a time t8, when the external input signal C_PH showing that the firstconversion process is being executed becomes L level, the switch circuitSW2 to set whether or not the first ramp voltage Vrc can be inputted isturned off, and the first conversion process is completed. In addition,when the external input signal C_PH shifts to L level at the time t8,the output level of the negative AND circuit 122 shifts from L level toH level in the control circuit 12, and the control signal Vctl to switchthe switch circuit SW3 from off state to on state is outputted from theDFF circuit 121 at the time of rising of the next clock signal SCLK (ata time t9).

At the time t9, when the switch circuit SW3 is turned on insynchronization with the rising of the clock signal SCLK, the secondconversion process is started. In the second conversion process, thesecond ramp voltage generator circuit 22 generates the second rampvoltage Vrf which monotonously decreases step-by-step by the secondchange amount ΔV2, in synchronization with the clock signal CLK of thedevice 1A of the present invention. The counter circuit 40 counts downfrom a maximum value of the lower-order bits by one in synchronizationwith the clock signal CLK, and outputs a digital value Cl [(y−1):0] (yrepresents the bit number of lower-order bits, or 2 in this embodiment)as a calculation result. Here, according to this embodiment, thelower-order bits are set to 2 bits, and ranges from 0 through 3, so thatthe maximum value of the lower-order bits is 3. In addition, a voltagelevel Vrfh (h=Cl) of the second ramp voltage Vrf is a value ofVrf0-ΔV2×C1 calculated by subtracting the second change amountΔV2×counter value Cl from the initial level Vrf0.

In addition, at the time t9, when the switch circuit SW3 is turned on,the third intermediate node N3 is configured such that the second rampvoltage Vrf is inputted through the switch circuit SW3 thereto as shownin FIG. 2. Therefore, the voltage level of the third intermediate nodeN3 at this time is at the initial level Vrf0 of the second ramp voltageVrf. After the time t9, while the switch circuit SW3 is in the on sate,the voltage level of the third intermediate node N3 is at the same levelas that of the second ramp voltage Vrf.

In addition, at the time t9, when the switch circuit SW3 is turned on,the voltage level of the second intermediate node N2 settles down to thevoltage level Vrck as shown in FIG. 3 because the voltage differencebetween it and the third intermediate node (Vrf0) is held. When thesecond conversion process is started, and the second ramp voltage Vrfwhich monotonously decreases step-by-step is inputted through the switchcircuit SW3, the voltage level of the second intermediate node N2monotonously decreases by the second change amount ΔV2 similar to thesecond ramp voltage Vrf.

At a time t10, the voltage level of the signal inputted to the voltagecomparator circuit CMP becomes smaller than the threshold voltage valueVth, and the output of the voltage comparator circuit CMP shifts from Llevel to H level. The counter value Cl [(y−1):0] (y represents thenumber of lower-order bits, or 2 in this embodiment) at this time is aconversion result of the lower-order bits, and stored in the lower-orderbit memory region 52. In addition, similar to the first conversionprocess, the time it takes for the level of the output signal Vcp of thevoltage comparator circuit CMP to be inverted differs according to thesolid-state imaging element PIX in the second conversion process.

Here, in the following formulas 7 and 8, the input voltage Vin of thevoltage comparator circuit CMP in the second conversion process in thecircuit 100A of the present invention is shown with the thresholdvoltage value Vth of the voltage comparator circuit CMP, the initiallevel Vrst of the solid-state imaging element PIX_(ij), the pixel levelVsig of the solid-state imaging element PIX_(ij) at the time of imagingoperation, the initial level Vrf0 of the second ramp voltage Vrf, andthe initial level Vrc0 of the first ramp voltage Vrc. In addition, thevoltage level Vrck is the voltage level of the first ramp voltage Vrc ina normal period after the output change of the voltage comparatorcircuit CMP in the first conversion process.

Vin=Vrf+(Vrck−Vrf0)+(Vsig−Vrc0)+(Vth−Vrst)  (7)

Vin−Vth

=Vrf−{Vrf0+(Vrst−Vsig)−(Vrck−Vrc0)}

=Vrf−{Vrf0−ΔV}

(ΔV=(Vrst−Vsig)−(Vrck−Vrc0)=Va−ΔVrc)  (8)

Therefore, the second ramp voltage Vrf at the time of the output changeof the voltage comparator circuit CMP in the second conversion process,that is, when Vin−Vth=0 is represented by the following formula 9, basedon the formulas 7 and 8 and FIG. 3. In addition, the voltage level Vrfincludes not only the voltage level of the second ramp voltage Vrf in anormal period but also the instantaneous voltage level at the time ofrising.

Vrf=Vrf0−ΔV

(provided that Vin−Vth)  (9)

Second Embodiment

A description will be made of a second embodiment of a circuit of thepresent invention and a device of the present invention, with referenceto FIGS. 4 to 6. In addition, according this embodiment, a descriptionwill be made of a case where configurations of the A/D converter unitand the first ramp voltage generator circuit are different from those inthe first embodiment.

First, a description will be made of configurations of the circuit ofthe present invention and the device of the present invention withreference to FIGS. 4 and 5. Here, FIG. 4 shows a schematic configurationexample of a device 1B of the present invention, on which a circuit 100Bof the present invention is mounted, and FIG. 5 shows a schematicconfiguration example of the circuit 100B of the present inventionaccording to this embodiment.

As shown in FIG. 4, similar to the first embodiment, the device 1B ofthe present invention is composed of, on the same chip, a solid-stateimaging element group IPD in which a plurality of solid-state imagingelements PIX_(ij) (i=1 to m, j=1 to n) are arranged in the form of amatrix, a plurality of A/D converter units 60 each provided with respectto each column of the solid-state imaging element group IPD, a memorycircuit 50, and those provided with respect to the solid-state imagingelement group IPD, such as a first ramp voltage generator circuit 23, asecond ramp voltage generator circuit 22, a control signal generatorcircuit 30, a counter circuit 40, a vertical decoder VD, and ahorizontal decoder HD. In addition, configurations of the solid-stateimaging element group IPD, the memory circuit 50, the second rampvoltage generator circuit 22, the control signal generator circuit 30,the counter circuit 40, the vertical decoder VD, and the horizontaldecoder HD are the same as those in the first embodiment.

The circuit 100B of the present invention according to this embodimentis a sub-ranging A/D converter circuit to execute a two-step conversionprocess composed of a first conversion process to find a value ofupper-order bits and a second conversion process to find a value oflower-order bits, and similar to the first embodiment, as shown in FIGS.4 and 5, it is composed of the A/D converter unit 60, the memory circuit50, the first ramp voltage generator circuit 23, the second ramp voltagegenerator circuit 22, the control signal generator circuit 30, and thecounter circuit 40.

As shown in FIG. 5, the A/D converter unit 60 is composed of a firstcapacitative element C1, a second capacitative element C2 having anoutput end connected to an input end of the first cap acitative elementC1, a third capacitative element C3 having an output end connected to aninput end of the second capacitative element C2, a voltage comparatorcircuit CMP having an input terminal connected to an output end of thefirst capacitative element C1 to compare a voltage value of an inputterminal with a predetermined threshold voltage value Vth, a first inputcircuit to input an externally inputted conversion object analog voltagesignal Vpix to a first intermediate node N1 between the firstcapacitative element C1 and the second capacitative element C2, a secondinput circuit to input a first reference voltage used for the firstconversion process to a second intermediate node N2 between the secondcapacitative element C2 and the third capacitative element C3, a thirdinput circuit to input a second reference voltage used for the secondconversion process to an input end of the third capacitative element C3,and a control circuit 62 to generate a control signal Vctl to hold avoltage value of the first reference voltage in the third capacitativeelement C3 at the time of output change of the voltage comparatorcircuit CMP in the first conversion process. In addition, configurationsof the first capacitative element C1, the second capacitative elementC2, the third capacitative element C3, the voltage comparator circuitCMP, and the first input circuit in this embodiment are the same asthose in the first embodiment.

The second input circuit according to this embodiment is composed of aswitch circuit SW4 to divide the second intermediate node N2 into afirst divided node ND1 on the side of the second capacitative element C2and a second divided node ND2 on the side of the third capacitativeelement C3, in order to control an electric connection between thesecond capacitative element C2 and the third capacitative element C3, aswitch circuit SW2 to set whether or not a first reference voltage canbe inputted to the first divided node ND1, and a switch circuit SW5 toset whether or not a third reference voltage such as the first referencevoltage or a voltage corresponding to the first reference voltage can beinputted to the second divided node ND 2.

In addition, the second input circuit according to this embodiment isconfigured to turn on the switch circuit SW2 in the first conversionprocess, turn off the switch circuit SW4 in the first conversionprocess, and keep the switch circuit SW5 in on state from the time whenthe counter circuit 40 starts counting in the first conversion processto the output change of the voltage comparator circuit CMP, based on thecontrol signal Vctl to hold the voltage value of the first referencevoltage value at the time of output change of the voltage comparatorcircuit CMP, in the third capacitative element C3.

In addition, according to the first embodiment, while the thirdcapacitative element C3 is electrically connected to the circuit toexecute the first conversion process when the voltage value of theconversion target of the second conversion process to find thelower-order bits is held in the third capacitative element C3, accordingto this embodiment, the second input circuit is provided with the switchcircuit SW4 and the switch circuit SW5, and the third capacitativeelement C3 is separated from the circuit to execute the first conversionprocess when the voltage value of the conversion target of the secondconversion process to find the lower-order bits is held in the thirdcapacitative element C3. Therefore, according to this embodiment, sincethe third capacitative element C3 is separated from the other circuit inthe first conversion process, a glitch noise which is generated when theswitch circuit SW5 is turned off does not spread to the other circuit,so that an image quality of a taken image of the device 1B of thepresent invention is effectively prevented from degrading.

The third input circuit according to this embodiment is configured insuch a manner that the second reference voltage is directly connected tothe input terminal of the third capacitative element C3.

As shown in FIG. 5, the control circuit 62 according to this embodimentis composed of a negative AND circuit 622 to calculate a negative ANDbetween an output signal Vcp of the voltage comparator circuit CMP, andan external input signal C_PH showing that the first conversion processis being executed, and a D flip-flop circuit (DFF circuit) 621 to latchand output an output signal from the negative AND circuit 622 when anexternally inputted clock signal SCLK rises. In addition, similar to thefirst embodiment, as compared with a clock signal CLK used to generatethe third reference voltage Vrc' in the first voltage generator circuit23 to be described below, the clock signal SCLK has the same frequencybut its phase is set to be shifted behind 3/4 cycle in view of asettling time of voltages of the input end and the output end of thecapacitative element C3. Thus, the switch circuit SW5 can be switched inthe first conversion process while the third ramp voltage Vrc' isstable.

According to this embodiment, the first ramp voltage generator circuit23 outputs a first ramp voltage Vrc and the same voltage as the firstramp voltage Vrc as the third reference voltage Vrc' in synchronizationwith the clock signal CLK.

Next, a description will be made of operations of the circuit 100B ofthe present invention and the device 1B of the present invention withreference to FIG. 6. Here, FIG. 6 shows signal waveforms and internalstate in the circuit 100B of the present invention according to thisembodiment.

According to this embodiment, in an initial state, a voltage value ofthe conversion object analog voltage signal Vpix is at a reset levelVrst, a voltage value of the first ramp voltage Vrc is at an initiallevel Vrc0, and a voltage value of a second ramp voltage Vrf is at aninitial level Vrf0. In addition, the switch circuits SW1, SW2, SW4, SW5,and SW6 are in off state.

In addition, configurations of the signal waveforms of the conversionobject analog voltage signal Vpix, the clock signal CLK, the clocksignal SCLK, the signal C_PH, the first ramp voltage Vrc, and the secondramp voltage Vrf are the same as those of the first embodiment. Inaddition, operations of the switch circuit SW1, the switch circuit SW2,and the switch circuit SW6 are the same as those in the firstembodiment. Therefore, the operations of the circuit 100B of the presentinvention until the first conversion process is started at a time t5 arethe same as those in the first embodiment, and at the time t5, thecapacitative element C1 holds a difference voltage between the resetlevel Vrst of the solid-state imaging element PIX as the reading targetand the inversion level Vth of the voltage comparator circuit CMP, andthe capacitative element C2 holds a difference voltage between theinitial level Vrc0 of the first ramp voltage Vrc, and a pixel level Vsigof the conversion object analog voltage signal Vpix.

At the time t5, when the external input signal C_PH showing that thefirst conversion process is being executed becomes H level, a samplingand holding process is started to hold the voltage used in the firstconversion process to find a value of the upper-order bits and thesecond conversion process. According to this embodiment, similar to thefirst embodiment, in the first conversion process, the first rampvoltage generator circuit 23 generates the first ramp voltage Vrc whichmonotonously increases step-by-step by a first change amount ΔV1 insynchronization with the clock signal CLK. In addition, similar to thefirst embodiment, the counter circuit 40 counts up from 0 by 1 insynchronization with the clock signal CLK, in response to the increaseof the ramp voltage, and outputs a digital value Cu [(x−1):0] (xrepresents the bit number of upper-order bits, or 2 in this embodiment)as a calculation result.

As shown in FIG. 6, at the time t5, when the external input signal C_PHbecomes H level, the output signal Vcp of the voltage comparator circuitCMP inputted to the negative AND circuit 622 and the external inputsignal C_PH both become H level in the control circuit 62, and theoutput level of the negative AND circuit 622 shifts from H level to Llevel. As a result, the output level of the control signal Vctloutputted from the DFF circuit 621 of the control circuit 62 is switchedat the time of rising (time t11) of the next clock signal SCLK, and theswitch circuit SW5 is turned on.

Here, since the switch circuit SW5 is turned on and the switch circuitSW4 is turned off, in the first conversion process according to thisembodiment, under the condition that the second capacitative element C2and the third capacitative element C3 are electrically separated fromeach other, the first reference voltage Vrc is inputted to the secondcapacitative element C2, the third reference voltage Vrc' is inputted tothe output end of the third capacitative element C3, and the secondreference voltage Vrf is inputted to the input end thereof. As a result,the operation regarding the first conversion process, and the operationregarding the sampling and holding process to hold the voltage used inthe second conversion process are prevented from being affected by eachother.

In addition, when the switch circuit SW5 is turned on at a time t11, asshown in FIG. 6, a voltage level of the second divided node ND2 becomesan initial level Vrc0′ of the third reference voltage Vrc′. Then, thevoltage level of the second divided node ND2 rises every time the thirdreference voltage Vrc′ rises. In addition, as shown in FIG. 6, since thevoltage waveform of the second divided node ND2 is gentle as comparedwith the voltage waveform of the first ramp voltage Vrc due to the onresistance of the switch circuit SW5 and the capacity value of thecapacitative element C3, the on resistance of the switch circuit SW5 andthe capacity value of the capacitative element C3 are to be designed inview of the settling time of the voltage of the second divided node ND2.

In addition, at the time t5, the first divided node ND1 is at theinitial level Vrc0 of the first ramp voltage Vrc because the first rampvoltage Vrc is inputted thereto through the switch circuit SW2. Inaddition, since the switch circuit SW2 is kept in on state in the firstconversion process, the voltage level of the first divided node ND1 alsorises every time the first ramp voltage Vrc rises. In addition, as shownin FIG. 6, since the voltage waveform of the first divided node ND1 isgentle as compared with the voltage waveform of the first ramp voltageVrc due to the on resistance of the switch circuit SW2, the onresistance of the switch circuit SW2 is to be designed in view of thesettling time of the voltage of the first divided node ND1. Since thefirst divided node ND1 (input end of the second capacitative element C2)and the second divided node ND2 (output end of the third capacitativeelement C3) are electrically separated from each other by the switchcircuit SW4 in the first conversion process, the first divided node ND1and the second divided node ND2 are prevented from being affected byeach other.

At a time t6, when a voltage level Vrck of the first ramp voltage Vrc,that is, the voltage level of the output end of the capacitative elementC1 becomes higher than the inversion level Vth of the voltage comparatorcircuit CMP, the output level of the output signal Vcp of the voltagecomparator circuit CMP is inverted from H level to L level. A countervalue Cu at this time is a conversion result of the upper-order bits,and it is stored in the upper-order bit memory region 51. In addition,while one A/D converter unit 60 is shown in FIG. 6, the time it takesfor the level of the output signal Vcp of the voltage comparator circuitCMP to be inverted differs according to the solid-state imaging elementPIX.

When the output level of the output signal Vcp of the voltage comparatorcircuit CMP is inverted to L level, the output level of negative ANDcircuit 622 becomes H level in the control circuit 62, and the DFFcircuit 621 outputs the control signal Vctl to turn off the switchcircuit SW5 at the time of rising of the next clock signal SCLK (timet7). Here, at the time t6, as shown in FIG. 6, the initial level Vrf0 ofthe second ramp voltage Vrf is inputted to the input end of thecapacitative element C3, and the voltage level Vrck of the thirdreference voltage Vrc′ is inputted to the output end of the capacitativeelement C3. When the switch circuit SW5 is turned off, a differencevoltage Vrck−Vrf0 between the voltage level Vrck of the third referencevoltage Vrc′ and the initial level Vrf0 of the second ramp voltage Vrfis held in the capacitative element C3.

At a time t8, when the external input signal C_PH showing that the firstconversion process is being executed becomes L level, the switch circuitSW2 to set whether or not the first ramp voltage Vrc can be inputted isturned off and the first conversion process is completed.

Then, at a time t12, the switch circuit SW4 is turned on for the secondconversion process, and the second conversion process is started. In thesecond conversion process, the second ramp voltage generator circuit 22generates the second ramp voltage Vrf which monotonously decreasesstep-by-step by a second change amount ΔV2, in synchronization with theclock signal CLK. The counter circuit 40 counts down from a maximumvalue (three in this case) of the lower-order bits by one insynchronization with the clock signal CLK, and outputs a digital valueCl [(y−1):0] (y represents the bit number of lower-order bits, or 2 inthis embodiment) as a calculation result. In addition, a voltage levelVrfh (h=Cl) of the second ramp voltage Vrf is Vrf0−ΔV2×Cl provided bysubtracting the second change amount ΔV2×counter value Cu from theinitial level Vrf0.

In addition, at the time t12, when the switch circuit SW4 is turned on,short circuit is caused between the first divided node ND1 and thesecond divided node ND2 of the second intermediate node N2 to beelectrically connected to each other. Therefore, as shown in FIG. 6,after the time t12, the voltage levels of the first divided node ND1 andthe second divided node ND2 become the same level. Since the switchcircuit SW2 is in off state and the switch circuit SW4 is in on state inthe second conversion process, the voltage levels of the first dividednode ND1 and the second divided node ND2 decrease every time the secondramp voltage Vrf inputted to the third capacitative element C3decreases.

At a time t10, the voltage level of the signal inputted to the voltagecomparator circuit CMP becomes smaller than the threshold voltage valueVth, and the output level of the voltage comparator circuit CMP shiftsfrom L level to H level. The digital value Cl [(y−1):0] (y representsthe bit number of lower-order bits, or 2 in this embodiment) at thistime is a conversion result of the lower-order bits and stored in thelower-order bit memory region 52. In addition, similar to the firstconversion process, the time it takes for the level of the output signalVcp of the voltage comparator circuit CMP to be inverted differsaccording to the solid-state imaging element PIX in the secondconversion process.

Here, according to this embodiment, it is desirable that the first rampvoltage Vrc used in the first conversion process and the third referencevoltage Vrc′ to generate the voltage used in the second conversionprocess are at the same level in the A/D converter unit 60. However,even when an offset voltage Ve (=Vrc0′−Vrc0) is generated in the thirdreference voltage Vrc′ with respect to the first ramp voltage Vrc, andan error is generated in the conversion results in the first conversionprocess and the second conversion process, the error is generated incommon in all the A/D converter units 60 because the third referencevoltage Vrc′ are inputted in common to all the A/D converter units 60,so that the offset component appears on the whole screen and it is notrecognized as a noise visually.

The following formula 10 and formula 11 show an input voltage Vin of thevoltage comparator circuit CMP in the second conversion process when theoffset voltage Ve is considered, in the circuit 100B of the presentinvention. In addition, a voltage level Vrck′ is a voltage level of thethird reference voltage Vrc′ during a normal period after the outputchange of the voltage comparator circuit CMP in the first conversionprocess. In addition, the voltage level Vrc0′ is the initial voltagelevel of the third reference voltage Vrc′.

$\begin{matrix}{{Vin} = {{Vrf} + \left( {{Vrck}^{\prime} - {{Vrf}\; 0}} \right) + \left( {{Vsig} - {{Vrc}\; 0}} \right) + \left( {{Vth} - {Vrst}} \right)}} & (10) \\\begin{matrix}{{{Vin} - {Vth}} = {{Vrf} - \left\{ {{{Vrf}\; 0} + \left( {{Vrst} - {Vsig}} \right) - \left( {{Vrck}^{\prime} - {{Vrc}\; 0}} \right)} \right\}}} \\{= {{Vrf} - \begin{Bmatrix}\begin{matrix}{{{Vrf}\; 0} -} \\{\left( {\left( {{Vrck} - {{Vrc}\; 0}} \right) - \left( {{Vrst} - {Vsig}} \right)} \right) -}\end{matrix} \\\left( {{{Vrc}\; 0^{\prime}} - {{Vrc}\; 0}} \right)\end{Bmatrix}}}\end{matrix} & (11)\end{matrix}$

Therefore, based on the formula 11 and FIG. 6, the second ramp voltageVrf at the time of the output change of the voltage comparator circuitCMP in the case where the offset voltage Ve is generated in the secondconversion process in the circuit 100B of the present invention isexpressed by the following formula 12. In addition,ΔV=(Vrck−Vrc0)−(Vrst−Vsig)=ΔVrc−Va, and the offset voltage Ve=Vrc0′−Vrc0

$\begin{matrix}\begin{matrix}{{Vrf} = {{{Vrf}\; 0} - \left\{ {\left( {{Vrck} - {{Vrc}\; 0}} \right) - \left( {{Vrst} - {Vsig}} \right)} \right\} - \left( {{{Vrc}\; 0^{\prime}} - {{Vrc}\; 0}} \right)}} \\{= {{{Vrf}\; 0} - {\Delta \; V} - {Ve}}}\end{matrix} & (12)\end{matrix}$

In addition, while the first ramp voltage generator circuit 23 isconfigured to output the first ramp voltage Vrc as the first referencevoltage, and the first ramp voltage Vrc as the third reference voltagein this embodiment, the present invention is not limited to this. Forexample, when the same first ramp voltage Vrc is used as the firstreference voltage and the third reference voltage, the first rampvoltage generator circuit 23 having the same configuration as that ofthe first embodiment may be used, and the first reference voltage Vrcmay be branched and inputted to the switch circuit SW2 and the switchcircuit SW5 in the A/D converter unit 60. In addition, when the firstramp voltage generator circuit 23 is configured to output the first rampvoltage Vrc as the first reference voltage and the first ramp voltageVrc as the third reference voltage to the electrically separated nodes,the noise of the first ramp voltage Vrc does not interfere with that ofthe third reference voltage Vrc′.

Third Embodiment

A description will be made of a circuit of the present invention and adevice of the present invention according to a third embodiment, withreference to FIG. 7B. In addition, according to this embodiment, adescription will be made of a case where a configuration of a secondramp voltage Vrf is different from that in the first embodiment and thesecond embodiment. Here, FIG. 7B shows the configuration of the secondramp voltage Vrf according to this embodiment, and FIG. 7A shows thesecond ramp voltage Vrf in the first embodiment and the secondembodiment to compare with the second ramp voltage Vrf according to thisembodiment.

First, a brief description will be made of configurations of a circuit100A of the present invention and a device 1 of the present inventionwith reference to the drawings. As shown in FIG. 1, similar to the firstembodiment, the device 1 of the present invention is composed of, on thesame chip, a solid-state imaging element group IPD, a plurality of A/Dconverter units 10 each provided with respect to each column of thesolid-state imaging element group IPD, a memory circuit 50, and thoseprovided with respect to the solid-state imaging element group IPD, suchas a first ramp voltage generator circuit 21, a second ramp voltagegenerator circuit 22, a control signal generator circuit 30, a countercircuit 40, a vertical decoder VD, and a horizontal decoder HD. Inaddition, according to this embodiment, configurations of components inthe device 1 of the present invention except for the second ramp voltagegenerator circuit 22, such as the solid-state imaging element group IPD,the A/D converter unit 10, the memory circuit 50, the first ramp voltagegenerator circuit 21, the control signal generator circuit 30, thecounter circuit 40, the vertical decoder VD, and the horizontal decoderHD are the same as those in the first embodiment. In addition, accordingto this embodiment, while a description will be made assuming that theconfigurations of the components in the device 1 of the presentinvention except for the second ramp voltage generator circuit 22 arethe same as those in the first embodiment, the configurations may be thesame as those in the second embodiment.

The circuit 100A of the present invention according to this embodimentis a sub-ranging A/D converter circuit to execute a two-step conversionprocess composed of a first conversion process to find a value ofupper-order bits and a second conversion process to find a value oflower-order bits, similar to the first and second embodiments, andsimilar to the first embodiment, as shown in FIGS. 4 and 5, it iscomposed of the A/D converter unit 10, the memory circuit 50, the firstramp voltage generator circuit 21, the second ramp voltage generatorcircuit 22, the control signal generator circuit 30, and the countercircuit 40.

The second ramp voltage generator circuit 22 according to thisembodiment is configured in such a manner that a total amount of thesecond change amounts ΔV2 (=ΔV2×8) in a period during which the voltagevalue of the second ramp voltage Vrf monotonously and sequentiallychanges step-by-step, that is, in a period between a time t21 and a timet24 in FIG. 7B, is set to be larger than the first ΔV1 (=ΔV2×4), and thesecond ramp voltage Vrf is generated and outputted in a period includinga given period just before the second conversion process (a periodbetween the time t21 and the time t22 in FIG. 7B) and a given periodjust after the second conversion process (a period between the time t23and the time t24 in FIG. 7B), in addition to the execution period (aperiod between the time t22 and the time t23 in FIG. 7B) of the secondconversion process.

More specifically, the second ramp voltage generator circuit 22according to this embodiment, as shown in FIG. 7B, generates the secondramp voltage Vrf which monotonously decreases by the second changeamount ΔV2 from a voltage level higher than the initial level Vrf0 bytwice the second change amount ΔV2, to a voltage level lower than theinitial level Vrf0 by the first change amount ΔV1 and twice the secondchange amount ΔV2. In addition, a voltage width in the period (the timet21 to the time t24) during which the second ramp voltage Vrfsequentially and monotonously decreases step-by-step is provided by thefollowing formula: ΔV2×2+ΔV1+ΔV2×2=ΔV2×8 in this embodiment, but it canbe optionally set.

Typically, when the difference voltage ΔV between a difference voltageVa between the reset level Vrst and the pixel level Vsig of theconversion object analog voltage signal Vpix, and the voltage level Vrckheld at the time of the output change of the voltage comparator circuitCMP in the first conversion process is extremely small or becomes anegative voltage (Va>Vrck), or when the difference voltage ΔV is higherthan the change amount (full-scale) of the second ramp voltage in theperiod during which the second conversion process is executed (the timet22 to the time t23), the input level of the voltage comparator circuitCMP does not reach the threshold voltage value Vth and its output levelis not inverted in the period between the time t22 and the time t23during which the detection result of the second conversion process canbe obtained. However, by configuring the second ramp voltage Vrf asdescribed above, even in such a condition, the input level of thevoltage comparator circuit CMP can reach the threshold voltage value Vthin the period between the time t21 to the time t24, so that conversionaccuracy can be effectively prevented from degrading.

Fourth Embodiment

A description will be made of a circuit of the present invention and adevice of the present invention according to a fourth embodiment, withreference to FIG. 7C. In addition, according to this embodiment, adescription will be made of a case where a configuration of a secondramp voltage Vrf is different from those in the first to thirdembodiments. Here, FIG. 7C shows the configuration of the second rampvoltage Vrf according to this embodiment, and FIG. 7A shows the secondramp voltage Vrf in the first embodiment and the second embodiment tocompare with the second ramp voltage Vrf according to this embodiment.

First, a brief description will be made of configurations of a circuit100A of the present invention and a device 1 of the present inventionwith reference to the drawings. As shown in FIG. 1, similar to the firstembodiment, the device 1 of the present invention is composed of, on thesame chip, a solid-state imaging element group IPD, a plurality of A/Dconverter units 10′ each provided with respect to each column of thesolid-state imaging element group IPD, a memory circuit 50, and thoseprovided with respect to the solid-state imaging element group IPD, suchas a first ramp voltage generator circuit 21, a second ramp voltagegenerator circuit 22, a control signal generator circuit 30, a countercircuit 40, a vertical decoder VD, and a horizontal decoder HD. Inaddition, according to this embodiment, configurations of the componentsin the device 1 of the present invention except for the second rampvoltage generator circuit 22, such as the solid-state imaging elementgroup IPD, the A/D converter unit 10, the memory unit 50, the first rampvoltage generator circuit 21, the control signal generator circuit 30,the counter circuit 40, the vertical decoder VD, and the horizontaldecoder HD are the same as those in the first embodiment. In addition,according to this embodiment, while a description will be made assumingthat the configurations of the components in the device 1 of the presentinvention except for the second ramp voltage generator circuit 22 arethe same as those in the first embodiment, the configurations may be thesame as those in the second embodiment.

Similar to the first and second embodiments, the circuit 100A of thepresent invention according to this embodiment is a sub-ranging A/Dconverter circuit to execute a two-step conversion process composed offirst conversion process to find a value of upper-order bits and asecond conversion process to find a value of lower-order bits, andsimilar to the first embodiment, as shown in FIGS. 4 and 5, it iscomposed of the A/D converter unit 10, the memory circuit 50, the firstramp voltage generator circuit 21, the second ramp voltage generatorcircuit 22, the control signal generator circuit 30, and the countercircuit 40.

The second ramp voltage generator circuit 22 according to thisembodiment is configured to generate the second ramp voltage Vrf whichmonotonously and sequentially changes. In addition, according to thisembodiment, the second ramp voltage Vrf is generated and outputted in aperiod including a given period just before the second conversionprocess (a period between a time t21 and a time t22 in FIG. 7C) and agiven period just after the second conversion process (a period betweenthe time t23 and the time t24 in FIG. 7C), in addition to the executionperiod (a period between the time t22 and the time t23 in FIG. 7C) ofthe second conversion process.

More specifically, the second ramp voltage generator circuit 22according to this embodiment, as shown in FIG. 7C, generates the secondramp voltage Vrf which monotonously decreases from a voltage levelhigher than an initial level Vrf0 by twice the second change amount ΔV2,to a voltage level lower than the initial level Vrf0 by the first changeamount ΔV1 and twice the second change amount ΔV2. In addition,according to this embodiment, as shown in FIG. 7C, while the second rampvoltage of ΔV2×2 is generated in each of the given period just beforethe second conversion process and the given period after the secondconversion process, the change amount of the second ramp voltage can beoptionally set. Similarly, while a voltage width in the period (the timet21 to the time t24) during which the second ramp voltage Vrfsequentially and monotonously decreases is provided by the followingformula ΔV2×2+ΔV1+ΔV2×2=ΔV2×8 in this embodiment, but it can beoptionally set.

By configuring the second ramp voltage Vrf to sequentially andmonotonously change, linearity of the second ramp voltage Vrf can bemore preferably provided. Since the second ramp voltage Vrf is superiorin linearity, the conversion accuracy can be preferably maintained whenit is necessary to set the resolution to a very high value in the secondconversion process. In addition, since a first ramp voltage Vrc is usednot only in the first conversion process to find the value ofupper-order bits but also in the sampling and holding process to holdthe voltage used in the second conversion process, it preferablymonotonously changes step-by-step.

ANOTHER EMBODIMENT

(1) While the clock signal CLK and the clock signal SCL having the samefrequency are used in the first conversion process and the secondconversion process in the descriptions of the first to fourthembodiments, a clock signal faster than a clock signal used in the firstconversion process may be used in the second conversion process.

In addition, as described above, the first ramp voltage (third referencevoltage) is used in the first conversion process to find the value ofthe upper-order bits and in the sampling and holding process to hold thevoltage used in the second conversion process, and the frequency of theclock signal to generate the first ramp voltage is determined in view ofthe settling time of the voltages at the input end and the output end ofthe capacitative element C3 used in the sampling and holding process.Meanwhile, the second ramp voltage is used in the second conversionprocess to find the value of the lower-order bits, and it is not used inthe sampling and holding process unlike the first ramp voltage.Therefore, as for the clock signal to generate the second ramp voltage,it is not necessary to consider the settling time of the voltage at theinput end and the output end of the cap acitative element C3 in thesampling and holding process, so that its frequency can be set to behigher than that of the first ramp voltage. When the frequency of theclock signal to generate the second ramp voltage is set to be higherthan that of the clock signal to generate the first ramp voltage, thetime taken for the second conversion process can be shortened, so that aprocess time of the A/D conversion process in the circuit of the presentinvention and the device of the present invention can be shortened as awhole.

(2) According to the first to fourth embodiments, the memory circuit 50is configured in such a manner that when the output level of the voltagecomparator circuit CMP shifts from H level to L level, the digital valueCu outputted from the counter circuit 40 is stored in the upper-orderbit memory region 51, and when the output level of the voltagecomparator circuit CMP shifts from L level to H level, the digital valueCl outputted from the counter circuit 40 is stored in the lower-orderbit memory region 52, but the present invention is not limited to theabove configuration.

It may be configured in such a manner that when the output level of thevoltage comparator circuit CMP shifts from L level to H level, thedigital value Cu outputted from the counter circuit 40 is stored in theupper-order bit memory region 51, and when the output level of thevoltage comparator circuit CMP shifts from H level to L level, thedigital value Cl outputted from the counter circuit 40 is stored in thelower-order bit memory region 52.

In addition, the memory circuit 50 may be configured in such a mannerthat one memory circuit provided with a memory region capable of storingdigital data for one row is provided, and the memory region of thememory circuit is divided into the upper-order bit memory region 51 andthe lower-order bit memory region 52 with respect to each column. Inaddition, the upper-order bit memory region 51 with respect to eachcolumn may be configured by combining the memory regions of theplurality of memory circuits, and similarly, the lower-order bit memoryregion 52 with respect to each column may be provided by combining thememory regions of the plurality of memory circuits.

1. An A/D converter circuit comprising: a first capacitative element; asecond capacitative element having an output end connected to an inputend of the first capacitative element; a third capacitative elementhaving an output end connected to an input end of the secondcapacitative element; a voltage comparator circuit having an inputterminal connected to an output end of the first capacitative element,configured to compare a voltage value of the input terminal with apredetermined threshold voltage value; a first input circuit configuredto input an externally inputted conversion object analog voltage signalto a first intermediate node provided between the first capacitativeelement and the second capacitative element; a second input circuitconfigured to input a first reference voltage to a second intermediatenode provided between the second cap acitative element and the thirdcapacitative element, the first reference voltage being used for a firstconversion process to find a value of sequential bits containing themost significant bit, in digital data provided after the conversionobject analog voltage signal has been converted; a third input circuitconfigured to input a second reference voltage to an input end of thethird capacitative element, the second reference voltage being used fora second conversion process to find a value of unconverted bits in thedigital data after the first conversion process; a control circuitconfigured to generate a control signal to hold a voltage value of thefirst reference voltage in the third capacitative element when an outputof the voltage comparator circuit changes in the first conversionprocess; a first ramp voltage generator circuit configured to generate afirst ramp voltage having a voltage value monotonously changingstep-by-step, and output the first ramp voltage as the first referencevoltage while at least the first conversion process is executed; asecond ramp voltage generator circuit configured to generate a secondramp voltage having a voltage value monotonously changing step-by-stepor sequentially, and output the second ramp voltage as the secondreference voltage while at least the second conversion process isexecuted; a counter circuit configured to output a digital value countedin response to a change of the first ramp voltage and a change of thesecond ramp voltage; and a memory circuit configured to store thedigital value when the output of the voltage comparator circuit changes.2. The A/D converter circuit according to claim 1, wherein the firstinput circuit includes a first switch circuit configured to set whetheror not the conversion object analog voltage signal is inputted to thefirst intermediate node, and turns on the first switch circuit in apredetermined period before the counter circuit starts counting in thefirst conversion process, the second input circuit includes a secondswitch circuit configured to set whether or not the first referencevoltage is inputted to the second intermediate node, and turns on thesecond switch circuit in the first conversion process, and the thirdinput circuit includes a third switch circuit configured to set whetheror not the second reference voltage is inputted to the thirdcapacitative element, and turns off the third switch circuit in a periodfrom when the output of the voltage comparator circuit changes to whenthe second conversion process starts, based on the control signal. 3.The A/D converter circuit according to claim 1, wherein the first inputcircuit includes a first switch circuit configured to set whether or notthe conversion object analog voltage signal is inputted to the firstintermediate node, and turns on the first switch circuit in apredetermined period before the counter circuit starts counting in thefirst conversion process, the second input circuit includes a fourthswitch circuit configured to divide the second intermediate node into afirst divided node on a side of the second capacitative element and asecond divided node on a side of the third capacitative element in orderto control an electric connection between the second capacitativeelement and the third capacitative element, a second switch circuitconfigured to set whether or not the first reference voltage is inputtedto the first divided node, and a fifth switch circuit configured to setwhether or not a third reference voltage such as the first referencevoltage or a voltage corresponding to the first reference voltage isinputted to the second divided node, and is configured to turn on thesecond switch circuit in the first conversion process, turn off thefourth switch circuit in the first conversion process, and turn on thefifth switch circuit in a period from when the counter circuit startscounting to when the output of the voltage comparator circuit changes inthe first conversion process, based on the control signal, to hold thevoltage value of the first reference voltage when the output of thevoltage comparator circuit changes, in the third capacitative element,and the third input circuit is configured to directly input the secondreference voltage to an input terminal of the third capacitativeelement.
 4. The A/D converter circuit according to claim 1, wherein thesecond ramp voltage generator circuit generates the second ramp voltagechanging in an opposite direction to a change in the first ramp voltagein the first conversion process, in the second conversion process. 5.The A/D converter circuit according to claim 1, wherein the voltagecomparator circuit includes an inverter circuit, and a sixth switchcircuit configured to cause short circuit between an input terminal andan output terminal of the inverter circuit, and short circuit is causedin the inverter circuit by the sixth switch circuit for initializationin a predetermined initialization period before the conversion objectanalog voltage signal is inputted to the first intermediate node by thefirst input circuit.
 6. The A/D converter circuit according to claim 1,wherein the control circuit controls an output timing of the controlsignal so as not to switch a switch circuit controlled by the controlsignal from when the first ramp voltage inputted to the thirdcapacitative element changes to when a voltage value changing inresponse to a change in the first ramp voltage becomes stable at theinput end and the output end of the third capacitative element, in thefirst conversion process.
 7. The A/D converter circuit according toclaim 1, wherein the first ramp voltage monotonously changesstep-by-step by a first change amount provided by dividing a voltagewidth of the conversion object analog voltage signal by a split numberbased on a first resolution previously set according to a bit number ofthe digital data, and the second ramp voltage monotonously changesstep-by-step by a second change amount provided by dividing the firstchange amount by a split number based on a second resolution defined sothat a sum of a value of a bit number corresponding to the secondresolution and a value of a bit number corresponding to the firstresolution is equal to a value of the bit number of the digital data. 8.The A/D converter circuit according to claim 1, wherein the first rampvoltage monotonously changes step-by-step by a first change amountprovided by dividing a voltage width of the conversion object analogvoltage signal by a split number based on a first resolution previouslyset according to a bit number of the digital data, and the second rampvoltage sequentially and monotonously changes.
 9. The A/D convertercircuit according to claim 1, wherein the second ramp voltage generatorcircuit generates and outputs the second ramp voltage in a periodincluding one or both of a period just before the second conversionprocess and a period just after the second conversion process, inaddition to an execution period of the second conversion process.
 10. Asolid-state imaging device comprising: a solid-state imaging elementgroup composed of a plurality of solid-state imaging elements arrangedin a matrix and configured to convert an optical signal to an electricsignal; a first capacitative element, a second capacitative element, athird capacitative element, a voltage comparator circuit, a first inputcircuit, a second input circuit, a third input circuit, a controlcircuit, and memory circuits provided with respect to each column of thesolid-state imaging element group; and a first ramp voltage generatorcircuit, a second ramp voltage generator circuit, and a counter circuitprovided on the same chip with respect to the solid-state imagingelement group, wherein the first capacitative element, the secondcapacitative element, the third capacitative element, the voltagecomparator circuit, the first input circuit, the second input circuit,the third input circuit, the control circuit, the memory circuits, thefirst ramp voltage generator circuit, the second ramp voltage generatorcircuit, and the counter circuit constitute the A/D converter circuitaccording to any one of claims 1 to 9.